JP4268614B2 - Automatic gain control circuit - Google Patents

Description

【Technical field】
[0001]
The present invention relates to an automatic gain control (AGC) circuit.
[Background]
[0002]
Conventionally, a circuit that keeps the amplitude of a video signal constant is known. According to this circuit, the difference between the pedestal level of a video signal including a synchronization signal (for example, a composite video signal, a Y signal of a Y / C separate signal) and the sync chip level, that is, the amplitude of the synchronization signal is constant. Kept. Such control is called sink AGC.
[0003]
That is, the ratio between the amplitude of the synchronizing signal and the amplitude of the video signal representing white is a value determined by the standard. Therefore, by controlling the amplitude of the synchronization signal to be constant, the amplitude of the white portion of the video signal is also maintained constant. For a video other than white, the amplitude of the video signal is proportional to the brightness of the video signal, and white has the maximum amplitude, so the amplitude of the video signal corresponding to the brightness of the video is uniquely determined.
[0004]
According to the ANSI / SMPTE 170M standard (NTSC standard), the ratio of the amplitude of the synchronization signal and the amplitude of the video signal expressing white is 2: 5. Another television signal standard ITU-R BT. In 470 (PAL standard), it is 3: 7.
[0005]
In addition to this, there is also known a method of maintaining the peak value of the video signal at a constant value, and this control is called peak AGC. Similarly to the sink AGC, the peak AGC forms a negative feedback loop to maintain the peak value of the video signal constant.
[0006]
The AGC circuit described in Japanese Patent Laid-Open No. 10-164458 is an example of a method in which a sink AGC and a peak AGC are combined, and the sink AGC and the peak AGC operate simultaneously. Among the input signals, there may be a video signal including an amplitude exceeding the maximum amplitude of the video signal determined by the standard (the amplitude of the white portion) from the amplitude of the synchronization signal. For this reason, the amplitude of the synchronization signal is smaller than the appropriate value, but the amplitude of the video signal may exceed the maximum amplitude. At this time, the operation for increasing the gain by the sync AGC because the synchronization signal is excessive and the operation for decreasing the gain by the peak AGC because the video signal is excessive are in a competitive state.
[0007]
In this state, the control becomes unstable (oscillates). For this reason, the time constant of the control loop of the peak AGC is set shorter than the time constant of the control loop of the sink AGC, thereby preventing the control system from becoming unstable (see FIG. 5).
[0008]
The AGC circuit described in Japanese Patent Application Laid-Open No. 2001-094826 employs a method classified as peak AGC. In this method, three types of threshold values L1, L2, and L3 (L1>L2> L3) are provided for the peak value of the video signal, and when L1 is exceeded, the excess is output as an error, which is larger than L2. When L1 or less, error 0 is output. When L1 is greater than L3 and L2 or less, a preset fixed error value is output. When L3 or less, error 0 is output.
[0009]
As a result, when the peak value of the video signal is excessive (greater than L1), the gain decreases, and when the video signal is at an appropriate level (greater than L2 and less than or equal to L1), the gain is maintained, and is too small (greater than L3 and less than L2). In this case, the gain increases, and when the video signal is close to black (L3 or less), the gain is maintained and excessive gain fluctuation is suppressed (see FIG. 4).
[0010]
In an apparatus that processes a given video signal and records or reproduces it, it is desirable that once the optimum gain is determined, the AGC gain does not vary as much as possible unless the source of the video signal is switched. .
[0011]
The sync AGC is based on the premise that the ratio of the amplitude of the synchronizing signal and the amplitude of the video is a predetermined value as described above, and has the advantage that it does not depend on the brightness of the video, and the above desirable characteristics can be easily achieved. Can be achieved.
[0012]
However, with regard to the video signal that is actually input, the signal is distorted due to the reception state of radio waves or recording on a video tape, and the ratio of the amplitude of the synchronizing signal to the amplitude of the video signal often changes.
[0013]
Further, when editing video, the sync signal may be changed, and in this case, the ratio of the amplitude of the video and the amplitude of the sync signal may be out of order.
[0014]
As described above, a signal in which the amplitude ratio between the video signal and the synchronization signal deviates from a predetermined value may be input. In many cases, a video signal having an amplitude exceeding the maximum amplitude of the video signal determined by the standard from the amplitude of the synchronization signal may be input. Therefore, with a simple sync AGC, even if the gain is appropriate for the synchronization signal, the video signal is converted into a video signal. On the other hand, the phenomenon that the gain becomes excessive occurs. When the gain of AGC is excessive with respect to the video signal, a large distortion occurs in the video signal, and the quality of the video deteriorates. The drawbacks of the sink AGC are also pointed out in the above prior art.
[0015]
On the other hand, the advantage of peak AGC is that AGC can be applied without a synchronization signal, and that the synchronization signal is appropriate and the video signal can be prevented from becoming excessive as described above.
[0016]
However, in the peak AGC, even if the amplitude of the input video signal is a peak value, this does not necessarily mean white, and there is no guarantee that it will be the maximum amplitude of the video signal in the future. Therefore, the determination of whether to increase or decrease the gain must eventually depend on the brightness of the surrounding image where the peak is detected, and for one signal source, the gain always varies according to the contrast of the image. . However, it is not desirable for the apparatus for recording and displaying the video signal that the gain fluctuates depending on the brightness of the video.
[0017]
Japanese Unexamined Patent Publication No. 2001-094826 proposes a method for improving this problem. This method determines that the amplitude of the video signal is appropriate, and there is a range in the amplitude of the video signal that maintains the gain. Furthermore, even in the case of a dark video, the gain control is stopped so that the gain does not vary depending on the brightness of the video. However, the range that can be determined to be appropriate varies depending on the video. For this reason, it is configured such that an appropriate level can be input from the outside, but it is unlikely that all images can be handled at a single level. Further, the disadvantage that the gain varies according to the brightness of the video signal cannot be completely eliminated.
[0018]
Japanese Patent Laid-Open No. 10-164458 proposes a method in which a sink AGC and a peak AGC are combined. In this method, the peak AGC is preferentially operated by making the time constant of the peak AGC shorter than the sink AGC in consideration of the time when the two controls compete with each other, and the stability of the control is also secured. It is an object.
[0019]
However, the case where the control of the two competes and the peak AGC is prioritized and the gain is limited so that the amplitude at the peak is not excessive is limited to when the video is bright. At this time, since the amplitude of the synchronization signal becomes smaller than the target value, the gain increases due to the normal sync AGC operation when the video becomes dark again. In other words, even with this method, it is impossible to exclude the possibility that the gain fluctuates depending on the contrast of the image at the same image signal source.
[0020]
As described above, an AGC circuit that solves the above problems and can maintain the amplitude of the video signal constant is desired.
DISCLOSURE OF THE INVENTION
According to one embodiment of the present invention, the video signal on which the sync signal is superimposed is input, the sync signal amplitude measuring means for measuring the amplitude of the sync signal through a variable gain amplifier, and the amplitude of the video signal An automatic gain control circuit having a video signal amplitude measuring means for measuring, for controlling the gain so as to keep the amplitude of the synchronizing signal measured by the synchronizing signal amplitude measuring means at a predetermined first reference value The first operation mode and the gain of the video signal measured by the video signal amplitude measuring means are not increased even if the amplitude of the synchronous signal measured by the synchronous signal amplitude measuring means is small, and the amplitude of the video signal measured by the video signal amplitude measuring means is a predetermined second And a second operation mode for reducing the gain only when the reference value becomes larger than the reference value.
[0021]
Thus, the amplitude of the video signal can be kept at a constant level while preventing the amplitude of the video signal from exceeding the predetermined second reference value without depending on the brightness of the video.
[0022]
Further, by switching the mode, the control target is limited to one, and there is no competition state of control due to having a plurality of control target values, so that stable control can be performed.
BEST MODE FOR CARRYING OUT THE INVENTION
[0023]
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0024]
FIG. 1 shows an embodiment of the present invention. In FIG. 1, the clamp circuit 102 sets a constant sync chip voltage for a video signal including a synchronization signal such as a composite video signal or a Y signal of a Y / C separate signal input to the input terminal 101. Maintain voltage (sink tip clamp). The variable gain amplifier 103 can change the gain by a control input, and amplifies or attenuates the video signal from the clamp circuit 102 with a gain determined by the AGC control unit 110. The A / D converter 104 converts the video signal from the variable gain amplifier 103 into a digital signal. The low-pass filter 105 removes the color signal included in the composite video signal from the A / D converter 104 and is necessary to prevent the synchronization separation circuit 106 from malfunctioning.
[0025]
The synchronization separation circuit 106 separates the composite synchronization signal from the signal that has passed through the low-pass filter 105. A horizontal AFC (automatic frequency control) circuit 107 estimates the position of the composite synchronization signal separated by the synchronization separation circuit 106 and outputs a signal indicating the position of the sync chip and the position of the back porch. The horizontal AFC circuit 107 is not necessarily required to determine the position of the sync chip or the position of the back porch. However, if the horizontal AFC circuit 107 is present, there is a risk of malfunction when the video signal contains noise. Becomes smaller. The sync signal amplitude measuring circuit 108 measures the level of the sync chip and the level of the back porch (that is, the pedestal level) with respect to the signal that has passed through the low-pass filter 105 every time a horizontal sync signal appears, and takes the difference between them. Thus, the amplitude of the synchronization signal is measured. The no-input detection circuit 111 receives the output of the horizontal AFC circuit 107 and determines whether or not a video signal is input to the input terminal 101. If it is input, it is a logic low level (hereinafter referred to as “L”). If it is not input, a logic high level (hereinafter referred to as “H”) is output.
[0026]
On the other hand, the video signal processing circuit 109 performs signal processing necessary for television signal processing such as Y / C separation and color demodulation on the digital signal from the A / D converter 104. The video signal processing circuit 109 includes a circuit that determines whether or not the amplitude of the video signal is larger than a predetermined second reference value.
[0027]
The AGC control unit 110 determines whether the video signal from the video signal processing circuit 109 is greater than a predetermined second reference value, and the amplitude of the synchronization signal from the synchronization signal amplitude measurement circuit 108, thereby adjusting the variable gain amplifier. The control value of 103 is generated, and the gain of the variable gain amplifier 103 is controlled.
[0028]
FIG. 2 shows a configuration of the AGC control unit 110 of FIG. In FIG. 2, an input terminal 201 is a terminal to which a signal indicating that the amplitude of the video signal is larger than a predetermined second reference value is input from the video signal processing circuit 109 of FIG. The input terminal 202 is a terminal to which a value representing the pedestal level from the synchronization signal amplitude measurement circuit 108 in FIG. 1 is input. The input terminal 203 is a terminal to which a value representing the sync chip level is input from the synchronization signal amplitude measurement circuit 108 of FIG.
[0029]
The low-pass filters 205 and 206 are for removing fine fluctuations of the pedestal level and the sync tip level that change in the horizontal period, and prevent the AGC gain from vibrating unnecessarily. An appropriate time constant is selected so that the outputs of the low-pass filters 205 and 206 become an average value of the pedestal level or sync chip level for the past one frame or one field.
[0030]
The time constant of the low-pass filter 204 is sufficiently smaller than that of the low-pass filter 205. As shown in FIG. 6, when the pedestal level applied to the input terminal 202 suddenly fluctuates, a difference occurs in the output of the subtractor 207 due to the difference in time constant between the low-pass filter 204 and the low-pass filter 205.
[0031]
The comparator 220 compares the absolute value of the output of the subtractor 207 with a predetermined third reference value, and determines that the signal has been switched if greater, and outputs H. The output 221 of the comparator 220 represents the signal switching as described above, and is connected to the mode management unit 211.
[0032]
The subtracter 208 obtains the amplitude of the synchronization signal by taking the difference between the pedestal level from the low-pass filter 205 and the sync chip level from the low-pass filter 206.
[0033]
The comparator 210 compares the amplitude of the synchronizing signal from the subtracter 208 with a predetermined first reference value, and when the amplitude of the synchronizing signal is larger than the predetermined first reference value, the output 216 becomes H. When the amplitude of the synchronization signal is smaller than the predetermined first reference value, the output 217 becomes H. When the amplitude of the synchronization signal is the same as the predetermined first reference value, the outputs 216 and 217 do not become H. . However, since the variable gain amplifier 103 (FIG. 1) used in the present embodiment is a digital control type and the gain setting is discrete, the output value of the subtracter 208 is the same as the predetermined first reference value. A range is given to the condition to be judged.
[0034]
The predetermined first reference value is a target value of the amplitude of the synchronization signal, and differs depending on the bit width of the A / D converter 104 or the signal standard (NTSC or PAL). In this embodiment, a 10-bit A / D converter is used, which is 224 for NTSC and 236 for PAL.
[0035]
The mode management unit 211 outputs the logic level of the output 217 of the comparator 210, the logic level of the input terminal 201, the logic level of the output 221 of the comparator 220, and the logic level of the output 222 of the no-input detection circuit 111 (FIG. 1). Based on the level, the logic level of the output terminal is set to H (representing the first operation mode) or L (representing the second operation mode). FIG. 3 shows the correspondence between the input of the mode management unit 211 and the operation mode. FIG. 7 shows the correspondence between the operation mode and the output of the mode management unit 211.
[0036]
The OR gate 212 performs an OR operation on the logic level of the input terminal 201 and the logic level of the output 216 of the comparator 210. The AND gate 213 performs an AND operation on the logic level of the output of the mode management unit 211 and the logic level of the output 217 of the comparator 210.
[0037]
The AGC gain control register 214 has two input terminals 218 and 219, and its output terminal is coupled to the gain control terminal of the variable gain amplifier 103 (FIG. 1). Here, when the output 218 of the OR gate 212 is H and the output 219 of the AND gate 213 is L, the value of the AGC gain control register 214 is decreased by 1, thereby decreasing the gain setting value by one step (gain is decreased). . On the other hand, when the output 218 is L and the output 219 is H, the value of the AGC gain control register 214 is increased by 1, thereby increasing the gain setting value by one level (gain is increased).
[0038]
In order to reduce the gain of the variable gain amplifier 103, it is necessary that the amplitude of the synchronization signal is larger than a predetermined first reference value or the video signal is larger than a predetermined second reference value. On the other hand, in order to increase the gain of the variable gain amplifier 103, it is necessary that the amplitude of the synchronization signal is smaller than a predetermined first reference value.
[0039]
Next, the operation of the AGC control unit 110 will be described with reference to FIG. In the initial state, the mode management unit 211 sets the logic level of the output to H and enters the first operation mode. Here, it is assumed that a signal having an amplitude smaller than 2/5 of the amplitude of the video signal representing the white color of the synchronization signal has entered, and the amplitude of the video signal is small as a whole. Then, the logic level of the input terminal 201 is L, the logic level of the output 216 of the comparator 210 is L, and the logic level of the output 217 is H. As a result, since the logic level of the output of the OR gate 212 is L and the logic level of the output of the AND gate 213 is H, the value of the AGC gain control register 214 is increased by one. As a result, the gain setting value is increased by one step, and thus the gain of the variable gain amplifier 103 is increased.
[0040]
This operation normally continues until the amplitude of the synchronization signal reaches a predetermined first reference value, but the ratio of the amplitude of the video signal representing white and the amplitude of the synchronization signal is greater than the value determined by the standard from the value determined by the standard. Therefore, the amplitude of the video signal exceeds the predetermined second reference value before the amplitude of the synchronization signal reaches the predetermined first reference value.
[0041]
When the amplitude of the synchronizing signal is smaller than the predetermined first reference value and the amplitude of the video signal is larger than the predetermined second reference value, the logic level of the input terminal 201 becomes H. As a result, the logic level of the output of the OR gate 212 becomes H. However, since the logic level of the output 219 of the AND gate remains H, the gain control directions compete.
[0042]
However, the mode management unit 211 indicates that the logic level of the output 221 of the comparator 220 is L and the logic level of the output 217 of the comparator 210 is H, but the logic level of the input terminal 201 has transitioned from L to H. Based on the above, the mode is switched from the first operation mode to the second operation mode, and the logic level of the output is set to L.
[0043]
When the logic level of the output of the mode management unit 211 becomes L, the output 218 of the OR gate 212 remains H, but the logic level of the output 219 of the AND gate 213 becomes L, so that the value of the AGC gain control register 214 Goes down by one. As a result, the gain setting value is lowered by one step, and thus the gain of the variable gain amplifier 103 is reduced.
[0044]
Thereafter, the state of the second operation mode is maintained as long as the amplitude of the synchronizing signal is smaller than the predetermined first reference value, and the current state is maintained as long as the amplitude of the video signal does not become larger than the predetermined second reference value. The gain setting is maintained. As a result, unnecessary flickering caused by gain fluctuations depending on the brightness of the screen can be prevented.
[0045]
When the amplitude of the synchronization signal does not become smaller than the predetermined first reference value during the operation in the second operation mode, the mode immediately shifts to the first operation mode, and the amplitude of the synchronization signal is the predetermined first reference value. The gain is controlled so that That is, a normal sync AGC operation is performed.
[0046]
As described above, the first operation mode and the second operation mode are provided, and the state where the amplitude of the synchronizing signal is smaller than the predetermined first reference value and the amplitude of the video signal is larger than the predetermined second reference value. When detected, the mode is changed to the second operation mode, and the gain is decreased until the state in which the amplitude of the video signal is larger than the predetermined first reference value is resolved. Then, after the amplitude of the video signal is attenuated to a predetermined second reference value or less, the gain setting is maintained, so that gain control independent of the brightness of the screen can be performed. That is, since there is no fluctuation in gain depending on the brightness of the screen, the flickering of the screen can be suppressed.
[0047]
In this embodiment, an example in which a digital control type is used as the variable gain amplifier 103 has been described. However, instead of the variable gain amplifier 103, a voltage control type variable gain amplifier is used, and the AGC control unit 110 is changed. Even if it is connected to this voltage controlled variable gain amplifier via a D / A converter, the same effect can be obtained.
[0048]
In this embodiment, an example in which the gain control operation is performed by a digital circuit has been described. However, similar gain control can be performed by using a microprocessor instead of the digital circuit.
[Industrial applicability]
[0049]
In the same video signal source, the amplitude of the video signal can be maintained at a constant level while preventing the amplitude of the video signal from exceeding a predetermined second reference value without depending on the brightness of the video.
Further, by switching the mode, the control target is limited to one, and there is no competition state of control due to having a plurality of control target values, so that stable control can be performed.
[Brief description of the drawings]
[0050]
FIG. 1 is a block diagram showing an embodiment of the present invention.
2 is a block diagram showing a configuration of an AGC control unit 110 in FIG. 1. FIG.
FIG. 3 is a table showing mode transition conditions in a table.
FIG. 4 is a diagram illustrating a relationship between an amplitude of a video signal and a threshold value in the related art.
FIG. 5 is a block diagram showing a configuration of a conventional AGC control unit.
FIG. 6 is a diagram illustrating an operation principle of a signal source switching detection unit.
7 is a diagram illustrating a correspondence between an operation mode and an output of the mode management unit 211 in FIG. 2;

Claims (3)

An automatic signal processing apparatus comprising: a synchronization signal amplitude measurement unit that inputs a video signal on which a synchronization signal is superimposed and measures the amplitude of the synchronization signal via a variable gain amplifier; and a video signal amplitude measurement unit that measures the amplitude of the video signal. A gain control circuit,
A first operation mode for controlling gain so as to keep the amplitude of the synchronization signal measured by the synchronization signal amplitude measurement means at a predetermined first reference value; and the synchronization signal measured by the synchronization signal amplitude measurement means The second operation mode for reducing the gain only when the amplitude of the video signal measured by the video signal amplitude measuring means becomes larger than a predetermined second reference value without increasing the gain even if the amplitude of the video signal is small A control means for controlling the variable gain amplifier ;
The amplitude of the synchronizing signal measured by the synchronizing signal amplitude measuring means during operation in the first operation mode is smaller than a predetermined first reference value, and the amplitude of the video signal measured by the video signal amplitude measuring means is When larger than a predetermined second reference value, the mode is switched to the second operation mode. On the other hand, the amplitude of the synchronization signal measured by the synchronization signal amplitude measuring means during operation in the second operation mode is a predetermined first value. An operation mode switching means for switching from the second operation mode to the first operation mode when it becomes smaller than a reference value;
An automatic gain control circuit comprising: 2. The automatic gain control circuit according to claim 1 , further comprising no-input detection means for detecting no-input of a video signal, wherein the operation mode switching means is not input by the no-input detection means during operation in the second operation mode. An automatic gain control circuit that switches from the second operation mode to the first operation mode when the signal is detected. 2. The automatic gain control circuit according to claim 1 , further comprising signal source switching detecting means for detecting switching of a video signal source, wherein the operation mode switching means is configured to switch the signal source during operation in the second operation mode. An automatic gain control circuit for switching from the second operation mode to the first operation mode when switching of the signal source is detected by the change detection means.

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